The present invention relates generally to acceleration detectors and relates more particularly to the use of such detectors as decelerometers for controlling electrically-operated axle brakes, as used for example for braking a towed vehicle such as a caravan coupled to a towing vehicle, such as a touring car.
In some caravans at least, braking is effected electrically, or more precisely electromagnetically, the control signal for which has to be a function of the deceleration of the towing vehicle, in accordance with the relevant standards. These standards now stipulate on the one hand that a control signal greater than zero should be sent to the brake for the immediate development of a braking couple as soon as the driver of the towing vehicle presses down the brake pedal, and on the other hand that there should then be a linear development of the braking couple provided by the brakes of the caravan as a function of the deceleration of the entire vehicle assembly consisting of the caravan and the towing vehicle.
Thus, the development of this braking couple should be controlled by a decelerometer.
In practice, a decelerometer of this kind is generally mounted on the towing vehicle and consists of a mercury decelerometer comprising a U tube containing mercury, in which the mercury level controls a set of photoelectric cells disposed vertically one above the other opposite a light source.
A mercury decelerometer of this kind has numerous disadvantages.
Firstly, the control signal which it generates is not in practice capable of linear development. Secondly, some adjustment is required as a function of the inclination of the towing vehicle and hence as a function of its loading, and finally, in spite of this adjustment, the signal delivered varies during braking, without any possibility of knowing the corresponding reference, owing to the variation in inclination which the towing vehicle is inevitably subjected to in the course of braking.
The use of other types of decelerometers has been proposed in order to avoid at least some of these drawbacks. This applies in particular to differential transformer decelerometers, i.e. decelerometers comprising a core which is movable relative to fixed windings, the response signal of which is, in practice, unaffected by the inclination of the towing vehicle. However, these decelerometers are expensive, and their response signals are not linear.
Similarly, because of their cost, decelerometers with a quartz strip and a bridge of strain gauges, as used in aeronautics, for example, are not suitable for use in controlling an axle brake for a trailer combination. Furthermore, their response range, which is relatively great, is not adapted to the much narrower range needed for control of this kind.
Finally, it has been proposed to use acceleration detectors of the kind comprising a movable member which is capable of moving against elastic return means which permanently urge it towards a stop, and a fixed member adapted to deliver an output signal as a function of the position of said movable member, said fixed member being a proximity detector capable of delivering a voltage proportional to the distance between itself and the movable member, e.g. a Hall effect sensor.
However, the constructions of this type known at present, if applied to the control of an electrically operated axle brake for a trailer combination, cannot comply with the standard which stipulates that a finite control signal should be sent to a brake of this kind as soon as the brake pedal of the towing vehicle is actuated.
The present invention relates to an acceleration detector capable of being used as a decelerometer which is modestly priced and is capable of meeting the relevant standards applying to electrically operated axle brakes for trailer combinations.